Turbulence intensity and turbulent transport characteristics behind gratings in pipes

Abstract

The magnitudes of the variance σ2 of the concentration distribution C(y) of a material being mixed in a stream behind turbulizing gratings in a pipe are measured. The impurity (helium) was inserted in the air stream via a thin pipe mounted directly behind one of the grating orifices. The intensity e of the turbulent stream velocity pulsations behind the grating was measured by the method of helium diffusion. Estimates of the magnitudes of the turbulent diffusion coefficient D and the diffusion scale of turbulencel were given on the basis of these measurements. Characteristics of the turbulent stream behind gratings in wind tunnels have been investigated in [1–3], for example. The process of turbulent diffusion behind gratings has been studied to a lesser degree. The flow directly behind the grating is of greatest interest for practice, but in the majority of papers the measurements have been made at a distance x from the grating which exceeds (10–20)M, where M is the distance, or “step,” between the centers of the grating orifices. Moreover, as a rule, the gratings investigated possessed low hydraulic drag and the intensity of stream turbulence ahead of them was considerably lower than ahead of the technical mixing apparatus.